How to attach files to emails on iPhone running iOS 8?

Here is my dilemma/situation:

Step1.

I download a PDF (or any other type) file from somewhere (i.e Safari, Email, Google Drive, etc.) into an application that can open it (i.e. Documents by Readdle)

Step 2.

I make some adjustments/edit the file using the app (i.e Documents by Readdle).

Step 3.

I go to the Email app and open an email that I want to reply to. I type up my message.

THE PROBLEM. (Ideally Step 4)

Now I want to attach that file I just edited in the app (i.e. Documents by Readdle) into this email reply that I am typing up.

How do I do this??

Any help would be appreciated.

Thanks